Method for recovery of expendable pattern material



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aired @tates This invention relates to a process for removing and recovering wax pattern material from investment casting molds made by the lost wax process.

In the lost wax investment casting process an expendable or removable wax pattern of the object to be cast is repeatedly coated with slurries of suspended refractory particles and stuccoed with dry refractory particles to form a multi-layer shell of refractory material around the wax pattern. The wax pattern is removed by melting, burning, or by solvents leaving a cavity of the desired configuration in the ceramic shell mold. The shell mold is fired to hardness and then receives molten metal.

The Wax removal step in the above-described casting proces is of critical importanc because the Wax pattern material must be removed without damaging the shell mold. A simple heating of the mold and pattern assembly to the melting point of the wax is unsatisfactory because the expansion of the wax material develops sufficient stresses to crack or break the relatively-thin shell mold before the wax melts and runs out the mold opening. It can be easily understood that severe cracking is cause for rejection of a mold.

It was found that heating of a mold and pattern assembly in hot air at from 200 to 500 F. caused cracking of the mold because of the pattern expansion. Heating at temperature around 2000 F., however, melted and burned the wax out through the shell mold without cracking because at that high temperature the wax is melted and burned before it can expand. Operating at such temperatures is costly and, of course, the wax pattern material is destroyed by burning.

Since it was known that shell molds are relatively porous, removal of the wax pattern material was attempted by suspending the mold and pattern assembly over vats of hot water, hot solvents, hot acids, or hot wax so that the vapors would rise around and through the mold to melt and remove at least that portion of the wax closest to the mold walls.

While the above-described wax removal methods would seem to be effective, the amount of mold failures due to cracking is still quite large. The uneven and extended heating of the thin shell mold itself causes cracks; and some of the vaporous agents adversely affect the mold materials. Additionally the very method of removing the wax, by vapors, causes sometimes an even greater expansion of the pattern material, and more cracks, than ordinary heating alone. Vapors of wax are not any more effective than vapors of other agents.

According to other wax removal processes to ensure a uniform and rapid heating of the mold, the mold and pattern assembly ar submerged in vats of hot water or solvents until the Wax is melted out. The rapid heating of the assembly has the advantage of melting the wax before it expands sufficiently to crack the mold. The wax, of course, must also be simultaneously removed as it is melted and the solvents are selected according to their ability to pass through the mold walls to dissolve and remove the wax. Such processes often take considerable time; and the selected solvents may damage the mold material.

While several of the above-described processes are suitable for a specific use with certain type molds and pattern materials, they all fail in one category essential for production line success of a casting process. In all of these processes, the wax pattern material is either lost by burning or contaminated with solvents, acids, etc. To successfully operate an investment casting process, it would be desirable to recover the comparatively expensive wax pattern material for reuse. Heretofore the small amounts of wax recovered had to be reprocessed to remove contaminants, that is, if its basic properties had not already been irreversibly changed.

It is the primary object of this invention, therefore, to provide a method for recovering as well as removing the waxpattern material from a shell mold.

It is also an object of this invention to provide a method for removing the wax pattern material from a shell mold without causing any cracking of the mold or unfavorable reaction with the mold material.

Other aims and advantages of this invention will be apparent from the following description and the appended claims.

In accordance with these objects a method is provided for removing and recovering for reuse the expandable pattern material. from a shell mold made around an expendable pattern comprising providing a bath of molten pattern material of substantially the same composition as the composition of the pattern material in the mold, immersing the shell mold and pattern assembly into the bath until substantially all of the pattern material melts, and removing the shell mold from the bath while allowing the pattern material to fall into the bath. It is not always necessary to completely melt all the pattern material. After the outer portions of the pattern have melted, the soft core may slip out of the mold into the bath. The wax reclaimed from the bath can thereupon be used to make more patterns. It is to be noted that the word expendable .does not mean here to be consumed in service, although that is often the case, but rather that the pattern material can be removed from the shell.

The temperature of the bath of molten pattern material should be substantially higher than the melting point of the material. Since waxes are different in basic composition, some having a petroleum base, others a vegetable base and many others synthetic in make-up, it is not possible to establisha standard bath temperature for all types of Waxes. However, the following considerations are to be used in determining the proper bath temperature: that bath temperatures only slightly above the melting point of the wax are undesirable because expansion of the wax pattern causes cracking before the pattern material is melted; that at these lower bath temperatures the wax bath has a higher viscosity hindering its circulation around and into the mold; at higher bath temperatures viscosity is decreased and circulation is increased as well as a higher rate of melting of the pattern material; at very high temperature th wax may decompose or even burn as its temperature reaches the flash point.

In accordance with these considerations several commercially available Waxes of different basic composition were tested to determine the preferred bath temperature when the given wax was used as a pattern material and as the bath material. The Waxes used are listed in Table I along with their melting points and recommended bath temperatures.

It is seen from Table I that there is no standard bath temperature or absolute rule for determining such a temperature. Wax G having a melting point of around 207 F. will effectively dewax at a minimum bath temperature of about 275 F., or about a 33 percent increase over the melting temperature; whereas wax E having a melting point of about 173 P. will dewax effectively at a minimum bath temperature of 350 F., or a percent increase in temperature over its melting temperature. It may be said in general that the minimum bath temperature for most waxes is between 200 F. and 300 3 F. The maximum bath temperature in all cases is the fiash point temperature of the wax, or temperature at which the wax begins to burn.

the Wax is dissolved out by vapors or solvents, the chemical contamination and dilution of the wax requires purification steps before reuse, if the wax can be used TABLE I Efiective Bath Temperatures for Selected Waxes Bath temperature range in degree Melting F. I Wax Composition point, F.

Broad Preferred A A petroleum parafiln Wax 131 to 34 225 up to flash point 325 to 400 13-.-. Blended commercial wax containing natural \vaxes- 143 to 154 275 up to flash point 325 to 400 petroleum resin and vegetable waxes. C A mlcrocrystalline paraffin wax 158 150 103 do 350 to 425 D Synthetic, halogenated hydroearbo 170 to 176 250 up to flash point.-. 325 to 400 E Synthetic commercial product 168 to 179 350 up to flash point.-- 375 to 500 12... A microcrystalline blended wax 163 to 1 8 250 up to flash point... 350 to 500 G Blended commercial product of having natural Wax 200 to 214 275 up to flash point. 300 to 400 base with synthetic wax additions.

The preferred bath temperatures for the given waxes are also shown in Table I. At these higher temperatures a melting and removal of the wax before expansion and cracking is guaranteed and also the removal process proceeds at a faster and more efiicient rate. In general it is seen that the preferred bath temperature for most waxes is from about 300 F. to about 500 F. provided the temperature is below the flash point and decomposition point of the wax.

The fact that wax removal proceeds at the comparatively low temperatures of 200 to 500 F. is surprising in view of known difficulties in removal of wax with hot air at such temperatures. A ceramic shell mold and pattern assembly heated in a furnace at 200 to 500 P. will crack because of the expansion of the wax pattern. Such molds must be heated at temperatures of about 2000 F. so that the wax will melt and burn through the pores of the shell before it can expand to crack the shell. It was thought before this invention that a wax bath would be ineffective in pattern removal because of the temperature limits imposed on the bath by the low flash point of wax. It was also felt that low-temperature dewaxing could be done only with vapors which easily permeatedthe shell mold to attack the pattern or with solvents and acids which combined melting with a chemical attack on the wax pattern. It is a surprising contribution to the art, therefore, that wax pattern removal can be done at the low temperatures taught by this invention.

The exact mechanism of the wax removal in a bath of wax kept at the relatively low temperatures of this invention without any cracking of the mold is not known. It is believed that, when the mold is submerged into the dewaxing bath, the liquid wax exerts a hydrostatic pressure inward toward the mold and pattern that tends to nullify the outward pressure of the expanding wax. As a result of this balance of forces, mold cracking is eliminated or substantially reduced. It is also thought that the liquid wax of the wax bath does not continually enter the mold through its porous shell to melt the pattern, but rather that the liquid wax uniformly heats the shell, the heat being conducted into the mold to melt the outer portion which is then drawn out of the mold by the presence of the liquid wax outside. In those processes where a bath of solvent or acid is used, the solvent may continually enter the pores of the shell to attack the wax pattern. It was felt before this invention that the use of a wax bath to remove a wax pattern would be ineifective in that wax was the material to be removed and not added to the porous molds by submersion in more wax.

Besides the efficiency of the wax removal effected by this invention the added advantage of recovering the wax pattern material constitutes a significant economic benefit. In those prior art processes where the wax pattern is burned out at high temperatures, the was is largely lost. And in those prior art processes where at all. By the process of this invention a very large portion of each wax pattern is recovered in a usable condition. After over one thousand molds were dewaxed by this process, a sample of reclaimed wax (wax F) had an ash or impurity content of only 0.133 percent by weight. The ash content may be further reduced by a filtering means in the bath to remove foreign particles. After the installation of a pump and filter having a 325- mesh (Tyler series) screen, the ash content of a similar wax after dewaxing over a thousand molds was only 0.098 percent by weight. The ash content of wax F before processing into patterns averages about 0.02 percent by weight. In addition ash contents considerably over 0.15 percent by weight may be tolerated especially when the particle size of contaminants is less than 325 esh.

The use of a pump to maintain circulation of the bath is especially advantageous.

As an example of the practice of the invention a number of mold clusters were dewaxed by the method described herein. Ten shell molds were formed around wax patterns of typical aircraft gas turbine blades. Each blade was approximately 6 inches long, 1 /2 inches wide and /2 inch thick at the base. There were several such blade patterns in each of the 10 shell molds.

The molds were prepared by dipping the blade assemblies into a ceramic slurry, and thereafter, while the coating was still wet, stuccoing dry refractory particles over the blades. After this coating dried, the process was repeated until a shell was produced about A to /2 inch thick. The mold was then permitted to dry. The molds had an overall size of about 6 by 7 by 10 inches. The bath material was wax F maintained at a temperature of about 425 F. The molds were immersed in the bath for six minutes and then removed for firing. During firing any excess wax not returned to the bath was burned out of the mold.

The following figures show the average wax recovery for 10 molds:

Mold weight before dewaxing 10.58 Mold weight after dewaxing 9.37 Recovered wax 1.21 Patternweight before investing 1.67 Efiiciency of recovery, percent 72.4

Approximate value of recovered wax per mold 0.53

may be now accomplished by first immersing the mold in a wax bath maintained at temperature below the melting point of the plastic so that only the wax is removed. The plastic pattern may then be safely removed by other processes, such as burning.

While it is preferred that the bath material and the pattern material be identical in composition, it is recognized that the bath material may contain one or more ingredients in a quantity that will not alter the characteristics of the bath or prevent its reuse. Additions to the bath may be made to obtain certain characteristics, for example, to modify the temperature range of the bath or to reduce the viscosity of the bath material.

The process may be made continuous to fit the needs of an assembly line by arranging molds with openings down on a traveling overhead support so that they dip into and travel through a trough of molten wax. After a trip through the wax sufficient to melt the wax, the molds are raised so that molten wax and globs of soft wax may run out into the bath. The molds can then travel on to the next processing step.

What is claimed is:

1. A method for removing and recovering for reuse the expendable wax pattern material from a shell mold made around an expendable pattern comprising providing a bath of molten pattern material of substantially the same composition as the pattern material in the mold immersing a shell mold and pattern assembly in the bath until substantially all of the pattern material melts, and removing the mold from the bath while allowing the pattern material to fall into the bath for reuse.

2. A method for removing and recovering for reuse the wax pattern material from a shell mold made around a wax pattern comprising providing a bath of molten wax of substantially the same composition as the wax pattern material in the mold, maintaining said bath at a temperature between about 200 F. and up to a temperature just below the flash point of the wax material, immersing a shell mold and pattern assembly in the bath until substantially all of the wax pattern melts, and removing the mold from the bath while allowing the wax pattern material to fall into the bath for reuse.

3. A method for removing and recovering for reuse the wax pattern material from a shell mold made around a wax pattern comprising providing a bath of molten wax of substantially the same composition as the wax pattern material in the mold, maintaining said bath at a temperature between about 300 F. and about 500 F., immersing a shell mold and pattern assembly in the bath until substantially all of the wax pattern melts, and removing the mold from the bath while allowing the wax pattern material to fall into the bath for reuse.

4. A method for removing and recovering for reuse the wax pattern material from a shell mold made around a Wax pattern comprising providing a bath of molten wax of substantially the same composition as the wax pattern material in the mold, maintaining said bath at a temperature of about 425 F., immersing a shell mold and pattern assembly in the bath until substantially all of the wax pattern melts, and removing the mold from the bath while allowing the wax pattern material to fall into the bath for reuse.

References Cited in the file of this patent UNITED STATES PATENTS 2,392,510 Stoody et al. Jan. 8, 1946 2,420,851 Zahn et al May 20, 1947 2,759,232 Demeter et al Aug. 21, 1956 2,790,219 Kohl et al. Apr. 30 1957 

1. A METHOD FOR REMOVING AND RECOVERING FOR REUSE THE EXPENDABLE WAX PATTERN MATERIAL FROM A SHELL MOLD MADE AROUND AN EXPENDABLE PATTERN COMPRISING PROVIDING A BATH OF MOLTEN PATTERN MATERIAL OF SUBSTANTIALLY THE SAME COMPOSITION AS THE PATTERN MATERIAL IN THE MOLD IMMERSING A SHELL MOLD AND PATTERN ASSEMBLY IN THE BATH UNTIL SUBSTANTIALLY ALL OF THE PATTERN MATERIAL MELTS, AND REMOVING THE MOLD FROM THE BATH WHILE ALLOWING THE PATTERN MATERIAL TO FALL INTO THE BATH FOR REUSE. 